A modern gas turbine aircraft engine may be subjected to extremes of operating attitudes and either negative or positive gravity conditions resulting from either the rotation of the engine nacelle relative to the airframe and/or flight maneuvers. It is typical to have a number of engine bearing compartments mounting bearings for the aircraft engine shaft and with the bearings being lubricated and cooled by oil delivered under pressure. The engine bearing compartment is sealed by labyrinth seals which are externally exposed to air under pressure which results in air in the engine bearing compartment in addition to the lubricating and cooling oil.
It is important to continuously remove lubricating and cooling oil from the engine bearing compartment to avoid the possibility of a flooded bearing compartment. The flooding of an engine bearing compartment would result in overheated bearings caused by the churning losses of flooded bearings or, in the worst case, an engine fire caused by oil leakage through the compartment shaft sealing system into hot sections of the engine.
The conventional solution to removal of lubricating and cooling oil is a lubrication scavenge system utilizing a pump having an inlet connected to an outlet at the engine bearing compartment. When the engine is subject to the extremes of operating attitudes and the negative and positive gravity forces, as previously mentioned, it is necessary to have two scavenge pickup locations at the engine bearing compartment wherein two outlets are located at different positions, with one scavenge pickup being located to withdraw lubricating and cooling oil under normal operation and the other outlet being positioned to withdraw lubricating and cooling oil under conditions other than normal operations, such as negative gravity forces. When operating under negative gravity forces, the lubricating and cooling oil collected in the sump of the engine bearing compartment as well as the air contained therein shift positions and the normal operation of one scavenge pickup being positioned to communicate with oil in the sump and the other to communicate with air in the sump is reversed. In normal operation, the fluid at the normal scavenge pickup will usually be a mixture of air and oil, while, at the other scavenge pickup, only air will be present. This relation reverses during negative gravity operation.
The lubrication scavenge system must operate effectively during normal operation as well as during negative gravity operation and transitions therebetween. The obvious and conventional solution to this problem has been to provide a lubrication scavenge system with two pumps for each engine bearing compartment with one pump being connected to the scavenge pickup outlet for normal operation and the other pump being connected to the other scavenge pickup outlet for negative gravity operation. It has been necessary to use two pumps to isolate the two scavenge pickup outlets from the engine bearing compartment from each other in order to assure that oil or a mixture of air and oil will always be pumped from the scavenge pickup outlet at which the oil is present.
The use of a pair of pumps to assure withdrawal of lubricating and cooling oil from the engine bearing compartment under all operating conditions increases the complexity, weight and size of the lubrication scavenge system because of the doubling of the pumping elements necessary to perform the required functions.
The prior art includes gear pumps having two or more intermeshing gears with plural inlets for pumping fluid from different sources. An example of such a gear pump having two intermeshing gears is the Behrends U.S. Pat. No. 3,420,180 wherein two intermeshing gears communicate with separate inlets for receiving fluid from two sources and discharging the fluid to a common outlet. The Behrends patent includes a bypass or ratio-change passageway extending from one of the pump inlets to the gear associated with the other pump inlet. Part of a mixture of air and oil can flow through the bypass passageway to join the flow of oil being pumped by the last-mentioned gear to assure a flow of cooling oil in a fixed ratio from two separate sources, with the two different fluids being pumped differing in resistance to the pump suction. The gear pump of the Behrends patent would not be effective in a lubrication scavenge system for an engine bearing compartment wherein one pumping element would be pumping air and the other pumping element would be pumping a mixture of air and oil, since the bypass passage would cross-connect the two pumping elements and there would be a leakage of a substantial amount of air to adversely affect the pumping of the mixture of air and oil.
Prior efforts to use a single pump with two inlets from an engine bearing compartment have not been successful, possibly for the reasons set forth in the Behrends patent in that the two fluids being pumped differ in resistance to the pump suction. An undesirably large proportion of one fluid may be pumped as compared to the amount of the other fluid being pumped.
The invention to be described hereinafter distinguishes over the prior art in providing a lubrication scavenge system for bearing compartments subjected to varying gravity conditions which utilizes a dual inlet gear pump that will pump simultaneously from two inlets any combination of gas, liquid or mixture thereof without interaction between the inlets or negative effects on the performance of either inlet and which continues to operate upon a change in fluids present at said inlets.